Photoelectrophoretic imaging method

ABSTRACT

A photoelectrophoretic ink is subjected to light and electric field a plurality of times during a single scanning pass over the ink by a plurality of web electrodes positioned within the boundaries of a narrow slit scanning light image.

United States Patent H 1 1111 3,719,484 Egnaczak 1 March 6, 1973 PHOTOELECTROPHORETIC IMAGING [56] References Cited METHOD UNITED STATES PATENTS [75 J Inventor: Raymond K. Egnaczak, Williamson,

N Y 3,553,093 1/1971 Putnam et a1 ..96/l.2 X 3,586,615 6/1971 Carreira ..204/18l PE X [73] Asslgneez Xerox Corporation, Stamford, Conn. Primary Examiner George F. Lesmes [22] Filed: Jan. 6, 1971 Assistant Examiner-John R. Miller Attorney-James J. Ralabate, David C. Petre and 1 P 104,333 Michael H. Shanahan [52 US. Cl ..96/1.2, 96/1 R, 96/13, ABSTRACT 204/181 355/4 355/8 A photoelectrophoretic ink is subjected to light and electric field a plurality of times during a single [51] Int. Cl. ..G03g 13/22 scanning pass over the ink by a plurality of web elec- [58] Field of Search ..96/ 1.2, 1.3, 1; 204/181 PE, trodes positioned within the boundaries of a narrow slit scanning light image.

9 Claims, 4 Drawing Figures PAIENIEDHAR'B 3,719,484

sum 10F 2 INVENTOR. RAYMOND K. EGNACZAK PATENTEDHAR "61913 3,719,484

SHEET 2 OF 2 FIG. 4

BACKGROUND OF THE INVENTION This invention relates to imaging systems and, in particular, to photoelectrophoretic imaging systems and apparatus.

Images are formed in the photoelectrophoretic process by exposing a photoelectrophoretic ink to electromagnetic radiation (hereafter referred to as light) and electric field. The ink is made of electrically photosensitive particles bearing a net electrical charge suspended in an electrically insulating liquid. Inks having different color particles are capable of producing full color images with conventional full color inks using cyan, magenta and yellow particles. An extensive description of the photoelectrophoretic process, materials and apparatus is given in U.S. Pat. Nos. 3,384,488; 3,384,565; 3,384,566; and 3,383,933, the disclosures of which are incorporated herein by reference. Briefly, the photosensitive particles are attracted to a transparent electrode by an electric field established between it and another electrode forming a dense layer of ink particles. This layer of particles is exposed to a pattern of light causing the light struck photosensitive particles to change charge polarity driving them from thetransparent electrode. The ink particles remaining on the transparent electrode comprise a positive image-of the light pattern and the exposed ink particles driven away comprise a negative image. The image and color quality of ink images produced from a polychromatic ink are improved by repeating the exposure and the'application of electric field. In simple system configurations using fiat plate transparent electrodes traversed by roller electrodes, the multiple exposure and application of field is not a particularly difficult requirement to meet. However, in more sophisticated systems wherein full frame light patterns are not used, the methods and apparatus for re-exposing an inkimage become relatively complex.

Accordingly, it is an object of the present invention to simplify the optics required for performing multiple exposures of a photoelectrophoretic ink.

More broadly, it is an object of this invention to improve the photoelectrophoretic imaging process.

Another object of the invention is to devise novel methods and apparatus for subjecting photoelectrophoretic ink and ink images to electric fields.

A specific object of the present invention is to devise methods and apparatus for performing multiple applications of light and field during a single scanning pass.

Another object of the invention is to devise method and apparatus for inhibiting corona currents between closely spaced electrodes.

- These and other objects of the instant invention are accomplished by positioning a plurality of web electrodes into a very narrow exposure region. The electrodes and exposure region are moved relative to a transparent electrode to form a full frame image on the transparent electrode in a line by line fashion during a single scanning pass. Exposure is made by projecting a light pattern through a narrow slit and through the transparent electrode to the photoelectrophoretic ink. The web electrodes include small rollers or squeegees positioned within the width of the exposure region as definedby the slit. Each roller has a web passed around it to carry away exposed particles driven toward it from 2 the transparent electrode. A liquid is usually applied to each web that is the same or similar to the electrically insulating liquid carrier in the ink. The dielectric properties of the liquid suppress corona currents in the vicinity outside the nips formed between the web and transparent electrodes.

DESCRIPTION OF THE DRAWINGS Other objects and features of the present invention will be apparent from a further reading of the description and from drawings which are:

FIG. 1 is a schematic, side elevation view of a photoelectrophoretic imaging system having a transparent drum electrode and continuous web electrodes DESCRIPTION OF THE EMBODIMENTS Polychromatic imaging systems of the present type normally include slit scan exposure optics for projecting a light pattern to the nip, i.e., interface, between two electrodes. The electrodes have large potential differences applied between them to establish afield sufficiently high for imaging. The slit scan optics is even used in systems having flat electrode shapes if images are made from opaque originals. The slit scan optics are used in the latter system because the light intensity required to flood a narrow strip is, far less than that needed for full frame image projection. Furthermore because image quality and color are improved with multiple exposures to light and field, the color system often includes means for projecting two or more slit scan images to two or more nips. An alternative to the simultaneous projection of multiple slit images is to recycle the scanning apparatus to make several scanning passes over an ink image. The present photoelectrophoretic system solves these and other problems by accomplishing a multiple exposure to light and field with a single scanning. pass of a slit image.

FIG. 1 illustrates the web electrodes of the present invention and how several of them are positioned within the scanning region to allow multiple imaging, i.e., multiple exposure to light and field, during a single scanning pass. The relative size of some of the elements are exaggerated to clarify their function. The drum 1 is a transparent electrode on which color ink images are formed from the photoelectrophoretic ink 2 contained in trough 3. The ink is applied to first imaging electrode 4 which carries the ink to the nip between it and the drum for the first exposure to light and field. A second and third exposure to light and field is made as the ink image on the drum moves past the second 5 and third 6 imaging electrodes. The troughs 8 and 9 contain electrically insulating liquids l0 and 1 I (normally the same as the insulating liquid of ink 2) which are carried to the nips between electrodes 5 and 6 and the drum to assist the migration of ink particles from the drum to an imaging electrode. The liquids l and 11 also act to suppress corona currents in regions of high field outside the nips.

A narrow image pattern is projected to the ink 2 by appropriate optical apparatus through the aperture or slit 13 formed between light stops 14 and 15. The width of the slit 13 substantially defines the imaging region which is small compared to the circumference of the drum 1. For example, when the circumference of drum 1 is in the order of 24-30 inches, the width of the aperture is in the order of 2$ to 2 inches, meaning that the exposed arc of the drum is relatively flat and any optical distortion due to curvature is negligible. Also, the width of the aperture is adequate for positioning at least two imaging electrodes opposite to it.

The ink image formed on the drum during a single rotation or scanning pass is transferred to a web 16 between the transfer electrode 17 and drum 1. The transfer electrode has a potential coupled to it to establish an electric field polarity to pull the ink particles comprising the image toward the web. The transferred image may be permanently fixed to the web by appropriate means.

The drum 1 is a complete 360 or partial transparent glass cylinder having a transparent conductive material (e.g., tin oxide) on its outside periphery. Conventionally, the conductive layer on the drum is electrically grounded with potentials substantially above and below ground being coupled to the imaging, transfer and other (e.g., a cleaning member) electrodes.

The imaging electrodes 4, and 6 are substantially identical, thereby requiring a detailed description of only one of them. The first imaging electrode 4 includes a thin conductive web 18 formed into a continuous loop supported between rollers 19 and 20. Roller 20 is coupled to an appropriate mechanical energy source in orderto rotate the web synchronously with the drum so as to establish a near zero relative velocity between the web and drum. Roller 20 is also coupled to an appropriate electrical energy source that maintains the web at a high potential. Web 18 has an insulating outer surface facing-drum l to prevent shorting the potentials coupled to the drum and web. The thickness of the insulating material is preferably small for high field in the nip between the web and drum and for high dielectric strength. The metal web may be comparatively larger for yielding mechanical strength to the web. Generally, the total web thickness is in the order of 1/32 to 1/64 inch and, consequently, is negligible compared to the width of aperture 13. V

A roller 19 is a small diameter roller journaled for rotation at the tip of a support block 25 that has -a cylindrical recess for supporting the roller. The small roller is not coupled to a power source but is rotated by the movement of a web 18. The diameter of a small roller 19 is generally of the order of is to V4 inch for the 24-30 in'ch drum mentioned earlier. With spacings of approximately if inch between electrodes 4 and 5 and between electrodes 5 and 6, the three electrodes, considering web thickness, roller diameter and electrode spacing, occupy a region approximately one inch wide.

The web configuration for the three imaging electrodes 4, 5 and 6 is significant because the webs carry away migrated ink particles to convenient locations for cleaning and wetting. The webs of the three electrodes converge to close spacing at the imaging region adjacent aperture 13 but they diverge to large spacings to permit cleaning and wetting at the troughs 3, 8 and 9. The brushes 26, 27 and 28 represent appropriate cleaning apparatus for removing ink particles from the surface of the electrodes. The cleaning function can be eliminated if consumable webs are used in place of the continuous webs. The wetting rollers 29, 30 and 31 represent appropriate apparatus for applying the liquids 10, 11 and 2, respectively, to the three electrodes 4, 5 and 6.

An alternative embodiment to that shown in FIG. 1 is to have inking apparatus similar to wetting roller 31 and trough 3 positioned to apply ink 2 directly to the drum 1 at a location counter clockwise from the aperture 13 but before the transfer roller 17. This configuration is helpful for simplifying the apparatus for cleaning the surface of electrode 4 because more space is available when the inking apparatus is not present.

Another alternative embodiment to that shown in FIG. 1 is to employ corotrons (electrostatic charging devices of the type described in U.S. Pat. No. 2,836,725) to establish the electric fields. The conductive webs 18 on the electrode 4, 5 and 6 are coupled to ground as well as the drum 1 and the corotrons are spaced from a web to deposit charge on the insulating surface of the web before it enters the nip. The charge is attracted to the grounded web whether an ink 2 or liquid 10 or 11 is on an electrode and is deposited in quantities such that the field in the nip is adequate for imaging to take place. This method of establishing the field is advantageous in that the thickness of the web can be greater for mechanical strength without fear of lowering field strength in the nip for practical potentials.

FIG. 2 illustrates a modified web electrode structure for a system like that shown in FIG. 1. Here only two electrodes are illustrated but more could be used. The idea in this embodiment is to expand the width of each individual nip between an imaging electrode and the drum. Imaging'electrodes 36 and 37 are composed of webs 38 and 39 similar to a web 18 being supported by a drive roller like a roller 20 but by two small idle rollers (rollers 40-43) instead of a small roller 19.

FIG. 2 illustrates another modification other than the additional number of small rollers 40-43. The other modification includes the meniscus forming dielectric wedge shaped members 44 and 45. The wedges are closely spaced from webs 38 and 39 to cause ink or liquid to form a meniscus that prevents air breakdown. The potentials coupled to the webs are sufficient to cause corona currents in regions between the webs and drum where separated by air or other ambient gas. The wedges are high dielectric constant materials that enable the majority of the field to be applied across it rather than the surrounding air. In addition, since the liquid fills the gap between the wedge and the web, air is excluded in regions outside the nip where the web and drum are closely spaced. At further spacings between the web and drum the field strength is insufficient to cause significant corona currents. The corona currents are undesirable at the entrance to an imaging electrode nip because it tends to reduce field strength, imparts undesirable polarities to the ink particles and other reasons. The wedge 45 between webs 38 and 39 is used principally to insure the space between rollers 41 and 42 is filled with liquid and/or ink. The webs are generally at the same potentials so the field is very low or zero in the spaces between the web 38 and 39 but the field is high in the space between rollers 41 and 42 close to the drum 1.

The electrode configuration of FIG. 2 offers great versitility for varying the angles 49-52 that the webs make with the drum, as compared to a horizontal reference line. The large angle 52 of web 38 as it enters the nip enables wedge 44 to be removed because the web and drum approach each other gradually. The air space at the entrance can be filled with a liquid for small spacings so that significant corona currents do not occur. On the other hand, angle 49 may be any desired angle that leads to gradual or abrupt separation between web 39 and drum 1 because the photoelectrophoretic process is not normally adversely affected by corona current generated at the exit to electrode nips. The angles 50 and 51 are generally selected for convenience and for insuring the presence of a liquid in the region close to drum 1 between rollers 41 and 42.

The velocity at which the slit image, projected through aperture 13, is moved past electrodes 4, 5 and 6 in FIG. 1 and electrodes 36 and 37 in FIG. 2, is selected to be substantially equal to the velocity of a point on the periphery of the drum 1.

FIG. 3 illustrates a photoelectrophoretic system wherein a flat transparent conductive electrode 56 has a slit image projected through it to the photoelectrophoretic ink 57. The aperture 58 formed between light stops 59 and 60 defines the width of the slit image which is preferably on the order of to 2 inches. The squeegee electrode assembly 62 includes a conductive body 63 and two or more (here four) squeegee blades 64-66 all spaced to fit within the imaging region defined by aperture 58. The insulating web 69 is woven around the squeegee blades 64-66 and pulled over the blades at a velocity that is substantially equal and opposite to the translational velocity of the assembly relative to the transparent electrode 56. The plate 56 is grounded and a high potential is coupled to the conductive body of assembly 62 giving rise to high electric fields in the nips between the tips of the squeegee blades and the transparent electrode.

An insulating liquid similar to that in ink 57 is applied to the web 69 before it enters the nips of the various blades to assist the migration of ink particles, minimize air breakdown and otherwise assist the multiple exposure process for the production of high quality images.

FIG. 4 is an enlarged view of squeege blade 65 which is typical of the other blades. The web 69 is wrapped around idle rollers 70 to reduce the mechanical drag on the web. The tip of each blade may include a compliant conductive member 70 such as conductive rubber to enable the web to be pushed closely to the transparent electrode generally uniformly along the length of the blade.

Whereas specific embodiments of the present invention have been given, it is apparent that modifications conventional to photoelectrophoretic imaging systems may be made without departing from the scope of the invention. The principal concept of this invention is the use of web electrodes in closelysgaced regions to accomplish multiple exposures to lig t and field during a single scanning pass.

What is claimed is:

1. A method of continuous imaging which comprises:

a. providing a first transparent conductive electrode;

b. coating photoelectrophoretic ink suspension of electrically photosensitive particles in an electrically insulating carrier liquid on said first electrode;

c. exposing said suspension on said first electrode to a slit scan light image projected through said electrode to form an imaging zone;

d. providing a first electrically insulating web backed by a second electrode;

e. bringing said first web into and out of contact with said suspension within said imaging zone while applying a potential difference between said first and second electrodes until an image is formed;

f. bringing at least one other electrically insulating web backed by an electrode into and out of contact with said imaging suspension while applying a potential difference between said electrode and said first electrode until said image is improved; and,

g. moving said webs and said first electrode in a manner to establish a substantially zero relative velocity between each of said webs and said first electrode.

2. The method of claim 1 wherein said webs include webs formed into continuous loops.

3. The method of claim 1 wherein said webs include webs not formed into continuous loops.

4. The method of claim 1 further including adding an electrically insulating liquid to a web prior to entering said nip.

5. The method of claim I wherein said transparent electrode includes at least a section of a cylinder.

6. The method of claim ll whereinsaid transparent electrode includes a flat plate.

7. The method of claim 6 wherein said slit light image is formed by steps including flooding a narrow region of an opaque original.

8. The method of claim 1 wherein an imaging electrode includes said web wrapped around a small diameter roller.

9. The method of claim 1 wherein said photoelectrophoretic ink is polychromatic. 

1. A method of continuous imaging which comprises: a. providing a first transparent conductive electrode; b. coating photoelectrophoretic ink suspension of electrically photosensitive particles in an electrically insulating carrier liquid on said first electrode; c. exposing said suspension on said first electrode to a slit scan light image projected through said electrode to form an imaging zone; d. providing a first electrically insulating web backed by a second electrode; e. bringing said first web into and out of contact with said suspension within said imaging zone while applying a potential difference between said first and second electrodes until an image is formed; f. bringing at least one other electrically insulating web backed by an electrode into and out of contact with said imaging suspension while applying a potential difference between said electrode and said first electrode until said image is improved; and, g. moving said webs and said first electrode in a manner to establish a substantially zero relative velocity between each of said webs and said first electrode.
 2. The method of claim 1 wherein said webs include webs formed into continuous loops.
 3. The method of claim 1 wherein said webs include webs not formed into continuous loops.
 4. The method of claim 1 further including adding an electrically insulating liquid to a web prior to entering said nip.
 5. The method of claim 1 wherein said transparent electrode includes at least a section of a cylinder.
 6. The method of claim 1 wherein said transparent electrode includes a flat plate.
 7. The method of claim 6 wherein said slit light image is formed by steps including flooding a narrow region of an opaque original.
 8. The method of claim 1 wherein an imaging electrode includes said web wrapped around a small diameter roller. 